1. Technical Field of the Invention
The present invention generally relates to a method of forming a thin film on a semiconductor device; more particularly, a method of forming a thin film on a semiconductor device using an atomic layer deposition (ALD) process.
2. Discussion of the Related Art
As semiconductor devices become more highly integrated, process conditions for forming a thin film on the semiconductor device, such as low heat budget, good step coverage, precise control for a thickness of the thin film, a low contaminated environment, etc., become more important.
The conventional deposition process, such as a low-pressure chemical vapor deposition (LPCVD) process, and plasma-enhanced chemical vapor deposition (PECVD) process are not suitable for forming a thin film on a state-of-the-art semiconductor device. For example, a conventional CVD process deposits a thin film at a relatively high temperature, which severely influences a semiconductor device, e.g., the redistribution of dopants in the substrate. In addition, the thickness of the thin film deposited by a CVD process varies across the surface of the semiconductor device. That is, the thickness of the thin film deposited around the surface area of a semiconductor substrate having a high density is thinner than the lower density surface areas of the semiconductor substrate, which causes a loading effect.
A thin film deposited through the LPCVD process (LPCVD thin film) contains a high percentage of hydrogen and has poor step coverage. On the other hand, a thin film deposited using a PECVD deposits a thin film at a relatively low temperature as compared with the LPCVD thin film, but it also has poor step coverage.
To avoid the above problems, an atomic layer deposition (hereinafter referred to as “ALD”) process, which can deposit a thin film at a relatively low temperature with good step coverage and without loading effect thereon, has been proposed.
U.S. Pat. No. 6,124,158 discloses an ALD process. According to the '158 patent, a first reactive material is introduced onto a treated surface, which means an operating surface of a semiconductor substrate that any treatment or process for manufacturing the semiconductor device is carried out, and a mono-layer is deposited on the treated surface by chemical reaction of the first reacting material. Then, a second reactive material is introduced and is chemically reacted with the treated surface to thereby form a desired thin film. After each of the above steps is carried out, a processing chamber in which the deposition process has been performed is purged of the reactive materials in order that the reactive materials are not chemically reacted with a remaining surface except the treated surface of the substrate.
When a thin film of silicon nitride (SiN) is deposited through the ALD process, the process temperature can be reduced up to about 100° C. as compared with the conventional processing temperature of 780° C. in the LPCVD process, and conformality of the thin film is excellent. Generally, a nitride layer, such as a Si3N4 layer, is usually used as a capping layer for protecting underlying layers due to an excellent diffusion barrier characteristic thereof. In addition, the nitride layer is also frequently used as an etch-stopping layer due to high etching ratio thereof. Accordingly, an etching ratio characteristic plays an important role when depositing the nitride.
However, even though a SiN layer deposited using an ALD process has good step coverage and a low processing temperature, there is a problem with the dry and wet etching characteristic of the deposited layer. The layer deposited by an ALD process is inferior to that deposited using the high temperature CVD process because of the relatively high hydrogen content in the layer. As a result, when the SiN thin film containing a high percentage of hydrogen is used as a spacer of a gate electrode of the semiconductor device, hydrogen atoms in the SiN thin film are diffused into a gate oxide layer by the heat budget in the process, which functions as an impurity trap and thereby deteriorating the characteristics of a transistor.
FIG. 1 is a graph showing the hydrogen content in the thin films deposited using various deposition processes. The hydrogen content in the thin films was measured by using FTIR-RAS (Fourier transform infrared reflection absorption spectroscopy). T350, T400, T450, T500, T550 and T595 mean that the ALD process was carried out at a temperature of 350° C., 400° C., 450° C., 500° C., 550° C. and 595° C., respectively. LP680 and LP780 mean that the LPCVD process was carried out at a temperature of 680° C. and 780° C., respectively. The PE-CVD means that the PECVD process was carried out.
As shown in FIG. 1, the hydrogen content in the SiN thin film deposited using an ALD process is higher than that of other SiN thin film deposited using a LPCVD at a high temperature of 780° C. As the design rule of a device pattern becomes narrower, a lower process temperature in the fabrication of the semiconductor devices is required.
U.S. Pat. No. 5,876,918 discloses a method of forming an insulating layer by using a gas without a chemical bond of nitride and hydrogen (N-H bond), such as a nitride layer deposited using a CVD process using nitrogen (N2) gas. However, the above method has a problem that the deposited layer is non-uniform in thickness and is of a poor quality.
In addition, there has been suggested a method of forming a nitride layer of low hydrogen content by using a nitrogen (N2) plasma gas or nitrogen (N) radical. However, when the plasma gas is directly applied onto the silicon substrate, the plasma gas increases the interface-state density in semiconductor device and fixes the charges in the nitride layer, thereby causing damage to the substrate.
A need therefore exists for a method using the ALD process, which can be carried out at a relatively low temperature, to deposit a thin film having a low hydrogen content or low hydrogen concentration in the deposited thin film.